Determining the mass of a vehicle and especially a utility vehicle is useful for many reasons. It can serve to determine the total weight of the vehicle and, for example, compare that with a highest admissible total weight. Furthermore, by subtraction of a known unladen vehicle weight or a total weight determined earlier, the weight of the load or the load change can be determined, and of course conversely, the total weight can be determined from the known unladen weight and a determined load. This enables evaluation related to legal provisions, such as in relation to a maximum admissible axle load or a maximum permissible total vehicle weight, and/or in relation to technical and design load limits.
Moreover, the value of the vehicle's mass or weight can also be needed as an essential input magnitude for control and/or regulation devices of the vehicle, which for example influence the operation of the drive motor, the transmission, the brake system and/or stabilizing devices. In vehicles with modern, automated manual transmissions it is desirable, for example, to select gear to be engaged, not only with reference to the known performance parameters of the drive motor and drivetrain, the vehicle's driving speed and the desired acceleration, but also as a function of the total mass of the vehicle, since if a vehicle is heavily loaded, substantially higher torques and therefore lower gears and higher engine speeds are needed for a desired acceleration, and in addition, for example uphill stretches also exert a greater influence on an optimum gear to be selected as the vehicle's mass increases, whereas the influence of wind gusts on the vehicle decreases as the vehicle's mass increases.
To determine the vehicle's mass it has long been known to use weighing devices external to the vehicle. However, these are often not available and are also relatively expensive and time-consuming to use, and/or they are at fixed locations and must therefore first be sought out, which is troublesome.
Furthermore, it is known for example from DE 100 58 045 B4 to determine the vehicle's mass with the help of devices on the vehicle itself, which rely on determining the weight force between the sprung and unsprung masses of the vehicle. More precisely, from the above document it is known, in the case of trucks, to measure the bearing load of the sprung vehicle parts directly or indirectly on all the axles or at least on several axles relatively far apart in relation to the vehicle's length, in order to determine from this the loading or, taking into account the known mass of the unsprung parts of the vehicle, the total weight of the vehicle. Of course, alternatively or in addition to this, and usually assuming a horizontal vehicle supporting area, the part-masses resting on each individual axle are determined, for example in order to determine the weight or mass distribution of the load.
Other solution approaches determine the supported weight at other points, for example, by evaluating the tire pressure or the pressure between a vehicle chassis and a container carrying the payload, such as a freight container, a liquid tank or a tilting container of a dump truck. The common feature of most such approaches is that a physical magnitude correlated with the mass to be determined is measured at several points geometrically separated from one another, such that the mass to be determined rests at least almost completely on the measurement points. For this purpose, for example, the pneumatic pressure in air-sprung elements of a front and a rear axle carrying the load can be determined.
In particular, for this purpose solutions are known, in which for example the pneumatic pressure at four points associated with the corners or the wheels of the vehicle is measured. To reduce the complexity of the apparatus required, simplified versions of this basic variant provide that for each axle or for each side of the vehicle, instead of separate pressure values only a mean pressure value is determined, since for example the left-hand and right-hand suspension of an axle or the front and rear suspension elements on one side of the vehicle are temporarily interconnected for measurement purposes.
If it is assumed as an approximation that under the usual measurement conditions the vehicle is about equally heavily loaded on each side of its longitudinal axis and is standing on a surface level enough for measurement purposes, it is sufficient for the pressure to be measured only on one side of the axles without interconnecting the suspension elements. Moreover, if the distribution of the carried load between several axles of an axle group is known, then instead of determining load values for all the axles of the axle group a measurement can be made on only one of the axles, and taking into account the assumed load distribution, the axle loads can be calculated from the value so obtained.
Such systems, which rely on a weight or pressure measurement while the vehicle is at rest, however, are subject to certain disadvantages in principle: when at least two force measurement devices are provided on different axles or at points of the vehicle remote from one another, this increases the cost and complexity of the equipment, namely the force measurement sensors, the wiring and the evaluation devices. As the number or force measuring devices increases this expenditure rises approximately proportionally, while at the same time the maintenance effort and the probability of failure increase. In addition, many trucks have leaf springs on their front axle, whose springs deform with increasing load not continuously but jerkily because of the friction between them, and so render any force measurement based on a path measurement of the spring deflection really inaccurate. However, the more the system is simplified, for example by taking physical measurement values only at individual axles or on one side of the vehicle, the more severely is the mass value determined on that basis affected by uncertainty and error.
From DE 10 2004 019 624 B3 an axle measurement unit for pneumatic and mechanical suspensions is known, which enables the total weight or the loading of a vehicle to be determined by measuring only one axle load or the bellows pressure of a pneumatic suspension bellows of one axle. For this purpose a load/sensor-signal diagram is determined, in that in a loaded and unloaded condition of the vehicle the bearing load determined by at least one sensor on an axle (for example the bellows pressure of a pneumatic suspension) is measured and correlated with known loads. By virtue of the support points of a characteristic line so determined, and on the assumption that the relationship is linear, any sensor value obtained or any bellows pressure of the pneumatic suspension bellows can be associated with a particular weight or load. To improve accuracy, additional support points can be determined so that even a non-linear relationship between the bellows pressure and the load can be represented with greater accuracy.
Compared with the systems described earlier, this known axle load measurement unit involves the least complex and costly equipment and therefore enables a determination of relevant vehicle masses sufficiently accurate for many purposes, for comparatively little cost. However, the determination of mass with the help of a weight measured on only one axle or even only one side of an axle is subject to major uncertainties in principle.
Finally, systems for determining vehicle mass are known, which are based on the evaluation of drive-dynamic parameters. Thus for example, according to a method disclosed in DE 198 37 380 A1 a traction force magnitude, in particular the time integral of the traction force, and a movement magnitude, in particular the change of speed, are determined during a traction-force-free phase and a traction-force phase of driving operation. This method has the advantage that no additional force or pressure sensors at all are needed, because all the raw data required are in any case available in suitable form in a modern motor vehicle for use in other vehicle systems. Accordingly, no or only very little additional wiring cost and complexity is involved and the probability of failure is independent of the number of axles.
This measurement variant, however, is especially advantageous in that with little cost it provides a signal that is very accurate in relation to the purpose intended: in particular, for the control of a transmission and motor it is not decisive how large the physical mass of the load is, but rather how large the driving resistance of the vehicle which, for example, results from a combination of the air resistance, the rolling resistance, the vehicle's mass and the inclination of the road are. In turn, the air resistance is a combination of the vehicle's air resistance coefficient, its cross-sectional area, the air density and the incident air flow speed, the latter itself depending on the driving speed and the wind situation at the time. The rolling resistance depends on the type and number of tires, the tire pressure, the suspension, the mass carried and the road surface properties.
The method proposed in DE 198 37 380 A1 takes all these components into account, in that only two relevant magnitudes are determined and set in proportion to one another, and is therefore in principle far superior to any method that seeks to control a drive motor and/or a transmission on the basis of a determined vehicle mass alone. The essential disadvantage of this method, however, is that at the moment of starting off after a substantial change in the loading of the vehicle, no mass change data related to this are yet available. Especially in the case of trucks this can restrict the utility of the method.
Essentially, in the case of trucks it can be assumed that if the loading or total vehicle weight is to be changed substantially, the vehicle must be at rest. True, there are special cases such as asphalt tippers or sprinklers, but in the latter case the weight reduction during operation takes place so slowly that a method such as the one described above can react to it without problems. Moreover, at least unless only small part-quantities of the load are concerned, loading and unloading are almost exclusively carried out while the vehicle's motor is switched off. Nevertheless, when starting off after the vehicle has been at rest, in most cases it is possible without problems to use the last vehicle mass value determined by a dynamic method for controlling the drive motor and an automated transmission.
Despite this, however, particularly once the vehicle's ignition has been turned off, the possibility exists that the previously determined vehicle mass or loading will differ so considerably from a value determined and stored earlier, that the starting-off process cannot be controlled or regulated with sufficient, or with the desired accuracy.